In a piezoelectric thin film resonator, a vibration part composed of a pair of excitation electrodes facing each other and a piezoelectric thin film provided therebetween must be acoustically separated from a substrate. Accordingly, some type of piezoelectric thin film resonator is formed so that a thin film member (membrane) partly floats over the substrate with an airspace layer provided therebetween.
In a piezoelectric thin film resonator having this type of structure, the thin film member includes support portions supported by the substrate and a floating portion floating over the substrate, the floating portion is supported by the support portions, a stress is liable to be concentrated on the vicinity of each boundary between the floating portion and the support portion, and as a result, cracking is liable to occur. In addition, a vibration part of the floating portion is liable to warp. Hence, the structure has been proposed in which a piezoelectric thin film is provided between a pair of excitation electrodes and is then further sandwiched with a pair of dielectric thin films (for example, see Patent Document 1: Japanese Unexamined Patent Application Publication No. 6-40611).
In the case in which a piezoelectric thin film is provided between a pair of excitation electrodes and is then further sandwiched with a pair of dielectric thin films, although cracking and warping of a vibration part can be suppressed, the vibration mode must be a harmonic vibration mode such as a third harmonic vibration mode, and as a result, the bandwidth of the resonant properties is decreased compared to that of a fundamental wave resonator. When this resonator is used, the filter bandwidth is also decreased, and hence a desired filter cannot be formed.
In addition, in a dicing step, because of insufficient strength of the vibration part, diaphragm damage is liable to occur. Furthermore, when the resonator is handled in mounting, since the vibration part is not covered for protection, a load is easily applied thereto, and hence elemental defects caused by damage are liable to be generated.